The toolkit pondered his second request. "I can’t see a way to do that without scribing something every bit as virulent as the Planck worms. It would have to mutate, itself, in order to deal with all the variants, and there’s no guarantee that it wouldn’t either burn out prematurely, or not at all."

Mariama said, "We can’t count on nine hours at the border. And if it falls again before we’ve finished the job, the next time can only be harder."

"So what do you suggest?"

"I’ve told you what I think we have to do," she said.

"Drop something through that can work from the inside? And I’ve told you what’s wrong with that. There are no magic bullets so smart that you can fire them into an uncharted world and expect them to repel an invader without destroying everything they’re meant to be saving." He laughed bitterly. "It’s hard enough believing that I can make those judgments myself."

"I know. Which is why you need to start making them from the other side of the border."

Tchicaya had suspected that this was where she was heading, when death interrupted her train of thought. He’d hoped to render the whole idea superfluous before she got around to putting it into words.

"You think I should send myself in?"

"The data rate would be fast enough. Seventeen minutes to build the interface, then about an hour to get you through."

"And then what? All our strategies for dealing with the Planck worms rely on correlating them with the vacuum. You can’t do that from the inside."

"So you look for other strategies," Mariama insisted, "once you’ve gone deep enough to have a better idea of what’s safe and what isn’t. I’m not saying we should give up working from this side, but there are advantages to both. A two-pronged attack can only improve our chances."

Tchicaya had run out of arguments. He looked up at his reflection in the window, knowing she could see it. "I can’t do this alone," he said. "I can’t go in there without you."

He waited for some scathing rebuke. This was even more self-indulgent than demanding that she pluck him from the vacuum, when he should have been willing to drift stoically into oblivion. The worst of it was, he still harbored doubts about her. How many chances to rid himself of her presence was he going to turn down?

Mariama said, "Joined at the hip, after four thousand years?"

"Joined at the kidney."

"I take it you won’t let me go in by myself?"

"No. Think of this as extending the old protocols for the Scribe. There always had to be an observer from the other faction, to keep everyone honest."

Tchicaya tried to keep his voice lighthearted, but this felt like the final recognition of the way it was between them. He had always followed her, every step of the way. Out of Slowdown, away from Turaev. Even in the centuries they’d spent apart, his own travels, his own adventures, had only seemed possible once she’d blazed the trail. He was not ashamed of this, but he wished he’d faced it squarely much sooner. He wished he’d told Rasmah, when the rebels first showed their hand: I am not the one to leave behind here. You head for the shuttle, I’ll head for the hub. Anyone can toss saboteurs from the scaffolding. But not everyone could walk into the far side alone.

Mariama said, "All right, I’ll go with you. We can keep each other honest. But the process has to be set up so it doesn’t jeopardize everything. If the border starts falling while only one of us is through, the vehicle will have to be programmed to interrupt the transfer, and dive without the second passenger."

"That makes sense," Tchicaya conceded.

"Which only leaves one thing to be decided."

"What’s that?"

"Who goes first."

Chapter 15

Tchicaya looked out from the Sarumpaet into a lime-green sea. In the distance, glistening partitions, reminiscent of the algal membranes that formed the cages in some aquatic zoos, swayed back and forth gently, as if in time to mysterious currents. Behind each barrier the sea changed color abruptly, the green giving way to other bright hues, like a fastidiously segregated display of bioluminescent plankton.